Generally, a high corrosion resistant metal pipe such as a stainless steel pipe has many advantages, but has a high unit cost of production resulting from use of a high priced material such as stainless steel, and has many difficulties in construction due to forming limitations in bending etc., and can be made straight only.
Also, a metal pipe manufactured in a straight shape has predetermined lengths for delivery, and an operation of connecting the metal pipes in a construction site requires considerable amounts of components, manpower, and time.
Also, when a metal pipe is buried in the ground, the metal pipe is inevitably susceptible to soil corrosion and electric corrosion etc.
Meanwhile, a resin pipe has a high corrosion resistance, a light weight, good constructability, and a low cost, but has a leakage risk due to separation of a connected portion caused by contraction and expansion with temperature changes and is vulnerable to pressure. Meanwhile, when manufacturing a resin pipe, extrusion is performed with an outer diameter of a resin pipe being slightly greater than a desired outer diameter, and the outer diameter is reduced through a sizing process during cooling to meet the density and surface requirements.
A metal resin composite pipe includes, as shown in FIGS. 1 and 2, a metal pipe 1 and a resin layer 5 formed on an outer surface of the metal pipe 1. A configuration and a manufacturing method of this metal resin composite pipe 10 is disclosed in Korean Patent No. 10-1094185.
The metal pipe 1 has a direct contact with a fluid flowing therethrough, and is made from a thin plate metal such as, for example, stainless steel, and thus has a high corrosion resistance. The resin layer 5 surrounds the metal pipe 1, and a thickness of the resin layer 5 is even greater than a thickness of the metal pipe 1. The resin layer 5 is made from a resin having a high corrosion resistance and a low cost. Accordingly, the metal resin composite pipe 10 has advantages of a high corrosion resistance to a fluid flowing therethrough, a high corrosion resistance to soils, and a low cost.
However, to deliver the metal resin composite pipe 10 to a construction site after manufacturing the metal resin composite pipe 10, the metal resin composite pipe 10 needs to be produced into a straight pipe having a predetermined length for the convenience of delivery, similar to a metal pipe. However, to use a straight pipe in a construction site, connecting the composite pipe 10 is required, and this connection operation involves considerable amounts of components, manpower, and time.
Accordingly, there is a need to manufacture the metal resin composite pipe 10 with a longer length while improving delivery performance.
To solve this problem, there was a need for production and supply of the metal resin composite pipe 10 by winding the metal resin composite pipe 10 circularly on a winder.
However, it is almost impossible to manufacture the metal pipe or the metal resin composite pipe 10 by winding in a ring shape due to characteristics of a material. To produce a pipe wound in a ring shape, development of a technique for winding the pipe while maintaining a circular cross section of the pipe is critical. Further, in view of storage and transportation of a product, minimizing the radius of curvature as much as possible while maintaining the circular cross section of the pipe was a problem that has to be solved. However, generally, when a bending force greater than or equal to an elastic limit is applied to the metal pipe to obtain a minimum curvature radius, a result is a deformation of the circular cross sectional shape or a folding of the pipe due to characteristics of steel, which cause a deformation of the pipe.
Meanwhile, as described in the foregoing, the metal resin composite pipe 10 is manufactured by coating an outer surface of the metal pipe 1 with a resin. The coating is implemented by a coating mold unit.
As shown in FIG. 3, a coating mold unit 20 includes an inner dice 21, an inner die lip 23 disposed at the rear of the inner dice 21, an outer die lip 25 disposed at the rear of the inner die lip 23, and an outer dice 27 surrounding the outer die lip 25.
The metal pipe 1 (not shown in FIG. 3) passes through the inner dice 21, the inner die lip 23, and the outer die lip 25 in a sequential order. An adhesive resin (not shown) is extruded on an outer surface of the metal pipe 1 through an adhesive resin injection hole 24a, and a resin is extruded through a resin injection hole 25a. 
Meanwhile, as described in the foregoing, when manufacturing a resin pipe, extrusion is performed with an outer diameter of a resin pipe being slightly greater than a desired outer diameter and the outer diameter is reduced through a sizing process during cooling to meet the density and surface requirements.
However, because the metal resin composite pipe 10 includes the metal pipe 1 embedded therein, the sizing process is infeasible, resulting in a low surface quality of the composite pipe 10. When an outer diameter of the composite pipe 10 is greater than an inner diameter S1, a resin flows back and remains in a carrier within an extruder. When the outer diameter of the composite pipe 10 is less than the inner diameter S1, an outer surface of the resin layer 5 fails to contact an inner wall of the outer die lip 25 so that a surface polishing effect is not obtained, and as a result, the resin layer 5 cannot have an proper density and the surface of the resin layer 5 becomes rough, which cause a poor quality.